A security/monitoring system is a field that has been researched for a long time and security and monitoring technologies in the related art include a security camera type, an IR type, an ultrasonic type, a sound measurement type, and the like.
U.S. Pat. No. 4,003,045 (Intrusion Detection Systems with Turbulence Discrimination, Stockdale, Roy Huntington NY US) discloses a method adopting the ultrasonic type. When there is no intrusion, upper and lower parts of an ultrasonic waveform are generated as the same amount and when intrusion occurs, turbulence of the waveform occurs. When the turbulent waveform is integrated through digital processing, it is possible to judge intrusion or no intrusion according to a degree of the turbulence waveform.
Since the method has a simple principle and transmits ultrasonic waves, and receives and processes the ultrasonic waves again by using an ultrasonic transducer, the method can be easily implemented. However, the method has a limit that no obstacle should be provided in the monitoring space. The reason is that when the ultrasonic waves start under a turbulent state by the existing object or condition, an abnormality in which an intrusion signal is generated occurs regardless of whether an intrusion or no intrusion occurs.
As another method, Korean Patent Application Laid-Open No. 2006-0128558 (Vehicle Ultrasonic Intrusion Detection Warning System and Method) is very similar to the above-mentioned first patent, but the application thereof is limited to the side of a vehicle. This method presents the vehicle ultrasonic intrusion detection warning system and method that can digitally modulate the ultrasonic waves, reduce misdetection by echoes or noise from the outside, and improve detection precision and further includes a technology that performs initialization setting through the amount of accumulated data in an initial setting step.
However, a disadvantage of the method is in that a dead zone is generated when the security space is extended to a wide space other than the vehicle because the ultrasonic waves are high in linearity. Further, this method has another disadvantage in that intrusion can be detected when omission of the ultrasonic waves are between an ultrasonic wave generator and an existing object is detected in the security space; however, intrusion at the back of the existing object cannot be detected. That is, security and monitoring in a space with an obstacle becomes difficult. Further, since a transfer function which is a unique characteristic of the monitoring space is not measured but variation to the initial setting is shown, it is difficult to overcome an influence by disturbance.
As yet another method, Korean Patent Application Laid-Open No. 2002-0056986 (Security device and method using environmental sound) discloses a method for monitoring whether an abnormality occurs in a monitoring place by using the environmental sound generated according to an environment of a predetermined place to be monitored. That is, a characteristic extracting unit extracts a predetermined characteristic from a sound signal by installing a microphone in the predetermined place to be monitored and an abnormality occurrence judging unit judges whether the abnormality occurs in the predetermined place by comparing the extracted characteristic with an environmental sound reference model stored in a memory, and a warning outputting unit generates a warning signal and warns a monitoring person when it is judged that the abnormality occurs.
A disadvantage of this method is that a maloperation possibility increases when the surrounding environment is quiet at the time of the initial setting, and since a sound state of a predetermined pattern is learned, it is very sensitive to surrounding noise after the initial setting through learning, such that a maloperation level is even larger when the surrounding noise is generated than in the intrusion state. Further, when an intruder or an intrusion object intrudes in state in which the surrounding environment is quiet, such that environmental sound does not vary, the method still maloperates even during intrusion.
As the last related art, Korean Patent Application Laid-Open No. 2009-0123752 (Low Power Security/Monitoring System Using Measurement of Sound Field Variation) discloses a method for judging intrusion or no intrusion in a predetermined space by using a difference between an initially set sound field and a sound field by the intrusion.
A disadvantage of the method is in that when intrusion is merely judged through the sound field difference, IF noise pollution is generated from the outside while measuring, and as a result, it is difficult to actually implement the method. In addition, when sound or noise is introduced from the outside, there is no method for solving an abnormality that occurs at the time of calculating the transfer function of the monitoring space.